Wastewater is typically collected and directed through a matrix of underground pipes that connect to the surface area through connections that are accessible by way of manholes. Liquids and solids can enter the sewer system through openings in the manhole covers, cracks in the piping, and in various other ways during construction activities and otherwise. When water seeps into the sewer pipes, the water can carry with it inert solid particles of various types. The solids can be as coarse as 4-25 mm gravel, but it is more common for sand sized at 0.10-2.0 mm and silt less than 0.10 mm to be carried into the sewer system by the water. The inert material that enters the sewer system in this way can range from less than 0.007 cubic meters per megaliter to more than 0.7 cubic meters per megaliter. Piping that is relatively old and systems having combined storm drainage and wastewater piping generally have the highest amount of grit.
The presence of grit in the wastewater that is collected for treatment has a number of harmful effects, including excessive wear on pumps and other equipment and blockages in pipes, valves, pumps and other parts of the sewage treatment system. When grit builds up to excessive levels in aeration basins or digesters, the basins or digesters must be taken out of service, and the costs to clean and repair the treatment system can be considerable. For these reasons, it has been recognized that grit should be removed at an early stage of the wastewater treatment process.
Standard grit separation practices make use of the high settling rates of the relatively dense grit particles. Grit settles at a higher velocity than most of the other particles in wastewater because the specific gravity of the grit is generally higher. Even so, organic particles that are relatively large can have settling rates that approximate those of finer grit-silt particles. Consequently, efforts to remove the finer grit particles also tend to remove at least the larger organic particles. The organics are removed with the grit because coarse organics (coffee grounds, for example) also have high settling rates. Also, organics can be entrapped by grit particles and settle with them.
The removal of grit larger than +65 or +100 mesh on the Tyler scale is desirable because it can drastically reduce the operation and maintenance costs of a wastewater treatment system. However, conventional aerated grit chambers are not always able to remove +65 mesh particles or even +35 mesh particles at times because of the way in which the grit chambers are designed.
Among the techniques used to remove grit are gravity settling in devices commonly known as “Detritors”, hydrocyclone degritting, cyclonic separation, and aerated grit chamber separation. The present invention is directed to improvements in the aerated grit chamber.
Past efforts to improve the performance of aerated grit chambers have included the provision of internal cross baffles and longitudinal baffles, airflow control, and the tapering of air. The baffles are used to reduce short circuiting of the flow through the central core of the grit chamber which lowers efficiency. However, even with the improvements that have been made, the grit chamber still is relatively inefficient over the wide ranges of influent flow that are encountered as a matter of course. It is common for the influent flow to vary during a 24 hour period between less than 40% of the average flow (during the night typically) to more than 250% of the average flow at peak periods.
A conventional grit chamber uses a rectangular basin in which air is distributed at one side to create a spiral flow pattern of water in the basin. Cross baffles and longitudinal baffles are often used to prevent short circuiting at the center. The basin is agitated heavily with water velocities of about 18-36 meters per minute. At these velocities, organics are washed from the grit particles. Multiple passes of the grit laden liquid across the floor causes the grit to be deposited on the floor or in a trough or hopper located in the floor. The collected grit is removed and further cleaned by hydrocyclones or other washing equipment.
In actual practice, the high energy levels resulting from the robust aeration has prevented aerated grit chambers from operating as efficiently as desirable. Grit particles of 0.210 mm and 0.149 mm size settle in a quiescent liquid at 1.32 meters per minute and 0.91 meter per minute, respectively. The actual liquid velocity of about 18-36 meters per minute is thus 14-40 times higher than the settling rate of these particles (which are 65 and 100 Tyler mesh particles).
One fundamental problem is that there is co-mingling of the turbulent aeration action with the gravity separation. Finer grit is lost with the effluent overflow by the high velocity currents that are created by the vigorous aeration forces. Particles as large as 20 mesh (0.847 mm) can be carried out of the grit chamber. There are a large number, that is, most of these conventional grit chambers currently operate without removing grit in an effective manner.
Inclined plate separators known as Lamella units have long been used in various types of equipment for separating solids from liquid and in other separation applications. Lamella units have been used in the clarification of raw sewage to accelerate the separation process of solids from the liquid. A plate separator of this type uses inclined plates to increase the effective area of the settling zone and thus enhance clarification while making efficient use of space. A plate separator can increase the effective area of the clarification by 400%-600% and can thus reduce the space to 20%-25% of what would otherwise be required without an adverse effect on performance. The degree of enhanced settling depends on the spacing between the plates, their angle of inclination, and their length.
Using a plate separator in an aerated grit chamber can enhance its effectiveness. Also, providing separate aeration and settling zones can enhance settling performance. However, the problem still remains of preventing the grit removal chamber from being overly efficient at low flow rates such that organics are removed with the grit. Excessive levels of organic removal in the grit chamber can result in a sludge buildup that creates septic conditions, odors, sludge flotation which plugs the separator plates and blocks flow through the grit chamber.